A wide variety of techniques have been developed to prepare and analyze biological specimens. Example techniques include microscopy, microarray analyses (e.g., protein and nucleic acid microarray analyses), and mass spectrometric methods. Specimens are prepared for analysis by applying one or more liquids to the specimens. If a specimen is treated with multiple liquids, both the application and the subsequent removal of each of the liquids can be important for producing samples suitable for analysis.
Microscope slides bearing biological specimens, e.g., tissue sections or cells, are often treated with one or more dyes or reagents to add color and contrast to otherwise transparent or invisible cells or cell components. Specimens can be prepared for analysis by manually applying dyes or other reagents to specimen-bearing slides. This labor-intensive process often results in inconsistent processing due to individual techniques among laboratory technicians.
“Dip and dunk” automated machines immerse specimens in liquids by a technique similar to manual immersing techniques. These automated machines can process specimens in batches by submerging racks carrying microscope slides in open baths. Unfortunately, carryover of liquids between containers leads to contamination and degradation of the processing liquids. Worse, cells sloughing off the specimen carrying slides can cause contamination of other slides in the liquid baths. These types of processes also utilize excessive volumes of liquids, resulting in relatively high processing costs when the reagents must be changed to reduce the possibility of specimen cross-contamination. Open containers are also prone to evaporative losses and reagent oxidative degradation that may significantly alter the concentration and effectiveness of the reagents, resulting in inconsistent processing. It may be difficult to process samples without producing significant volumes of waste that may require special handling and disposal.
Immunohistochemical and in situ hybridization staining processes are often used to prepare tissue specimens. The rate of immunohistochemical and in situ hybridization staining of sectioned fixed tissue on a microscope slide is limited by the speed at which molecules (e.g., conjugating biomolecules) can diffuse into the fixed tissue from an aqueous solution placed in direct contact with the tissue section. Tissue is often “fixed” immediately after excision by placing it in a 10% solution of formaldehyde, which preserves the tissue from autocatalytic destruction by cross-linking much of the protein via methylene bridges. This cross-linked tissue may present many additional barriers to diffusion, including the lipid bilayer membranes that enclose individual cells and organelles. Conjugate biomolecules (antibody or DNA probe molecules) can be relatively large, ranging in size from a few kilodaltons to several hundred kilodaltons, which constrains them to diffuse slowly into solid tissue with typical times for sufficient diffusion being in the range of several minutes to a few hours. Typical incubation conditions are 30 minutes at 37 degrees centigrade. The stain rate is often driven by a concentration gradient so the stain rate can be increased by increasing the concentration of the conjugate in the reagent to compensate for slow diffusion. Unfortunately, conjugates are often very expensive, so increasing their concentration is wasteful and often not economically viable. Additionally, the excessive amount of conjugate that is driven into the tissue, when high concentrations are used, is entrapped in the tissue, is difficult to rinse out, and causes high levels of non-specific background staining. In order to reduce the noise due to non-specific background staining and increase the signal of specific staining, low concentrations of conjugate with long incubation times are often used to allow the conjugate to bind only to the specific sites.
Histology staining instruments often use relatively large volumes of reagent (100 μL) in a puddle of typically 300 μL of buffer. Some conventional instruments mix the reagent by alternating tangential air jets onto an overlaying oil layer that rotates and counter-rotates when contacted by the alternating air jets, thereby imparting motion into the underlying aqueous puddle. This mixing is slow and not particularly vigorous, and it can create significant evaporation losses, especially at the elevated temperatures that are often necessary. Large volumes of rinse liquid are used to physically displace the large puddles of reagents, which are covered with oil. This rinsing procedure produces large volumes of waste liquid, which may be hazardous waste.